The invention relates generally to semiconductor device fabrication and, in particular, to device structures for a trench anti-fuse, and to design structures for programmable integrated circuits.
Anti-fuses are nonvolatile, one-time programmable devices widely used in various programmable integrated circuits. An anti-fuse structure, which comprises a pair of conducting terminals separated by a dielectric layer, is connected to an integrated circuit and is initially non-conductive at the time of fabrication. When it is desired to change the operational configuration of the integrated circuit, an anti-fuse may be irreversibly programmed to create a permanent conductive link that closes a previously open, high resistance circuit path. One common use of anti-fuses is in redundancy circuits of dynamic random access memories and static random access memories. Replacing a defective circuit element typically entails blowing an anti-fuse to program a redundant circuit element to respond to the address of the defective primary circuit element in the memory device. Another common use of anti-fuses is in programmable read-only memories and programmable logic devices (PLDs) to program logic circuits to create a customized design. Yet another common use of anti-fuses is to program the I/O configuration of a memory device.
Specifically, application of a stimulus, such as suitable electrical current passed through the dielectric layer by application of a predetermined voltage to the pair of terminals, operates to break down the dielectric layer and thereby to significantly reduce the electrical resistance of the dielectric layer. The reduced electrical resistance of the dielectric layer creates a closed conductive link or short between the terminals. Once programmed to provide the low-resistance, closed state, the anti-fuse cannot be programmed back to a high-resistance, open state. Programming voltages for planar anti-fuse structures are significantly greater than five volts, which makes existing anti-fuse structures incompatible with advanced integrated circuit designs.
Fabricating anti-fuses in trenches increases the device density in comparison with conventional planar anti-fuse device structures. However, when blown, trench anti-fuses may often exhibit poor breakdown uniformity because the trenches are characterized by different cross-sectional geometrical shapes at different trench depths. The non-uniformity in cross-sectional shape with depth arises from the crystallographic orientation dependence of trench etch processes like reactive ion etching. For example, a shallow portion of a trench may exhibit a substantially octagonal cross-sectional geometrical shape and a deeper portion of the same trench may exhibit a substantially rectangular cross-sectional geometrical shape. The thickness of silicon oxide grown on the trench sidewalls also exhibits a dependence upon crystallographic plane. For example, silicon oxide grows thicker on (110) crystal planes of silicon than on (100) crystal planes. Silicon oxide also grows significantly thinner at trench corners than on the trench planes between adjacent corners. Furthermore, it is very difficult, if not impossible, to control the curvature of trench corners formed by reactive ion etching processes.
The uniformity of programming operation of a trench anti-fuse is primarily determined by the oxide thickness and the electric field at the trench corners. The electric field is determined primarily by the curvature of the trench corners. During the operation programming a trench anti-fuse, only the thin oxide at trench corners in the deeper portion of the trench typically breaks down to provide the low-resistance, closed state connecting the terminals. The non-uniformity in trench shapes and corner curvatures leads to undesired large variation in the characteristics of trench anti-fuse structures. This adversely affects the performance and predictability of the anti-fuse structure. Furthermore, conventional trench anti-fuses still require a programming voltage significantly in excess of five volts to accommodate the large observed variation in anti-fuse characteristics.
Improved device structures and design structures are needed for anti-fuses that alleviate these and other problems associated with conventional anti-fuse device structures and design structures.